An industrial furnace, such as a tubular furnace, metal heating furnace, ceramic industrial kiln, metal melting furnace, gasification melting furnace, boiler or a combustion heating type of heating apparatus, such as a radiant tube burner, is provided with fuel feeding means for feeding hydrocarbonaceous fuel, air supply means for supplying combustion air, and a combustion means for mixing the fuel and combustion air to burn the fuel, such as a burner. The fuel and combustion air mixed in the combustion means produce flame in a combustion area by diffusion combustion. In this kind of combustion means, the actual amount of combustion air is set to be an excess air ratio exceeding a theoretical amount of air for the fuel, and the mixing ratio of combustion air and fuel (air-fuel ratio) is, in general, set to be approximately 14˜15. Generally, pre-mixing of fuel and combustion air before fed to a burner is not adopted, because of its possibility of unexpected back fire, and therefore, the combustion air and fuel are introduced into a burner throat or in-furnace area through an air delivery port and fuel injection port so as to be mainly mixed in a proximal zone of the burner. For instance, a burner is provided with a flame stabilizer of swirler type, flame holder type or the like, in order to desirably mix a fuel injection flow and an air flow having different flow rates. The flame stabilizer causes an ignitable high-temperature circulation flow in the mixing area of fuel and air, whereby it prevents blow-off of flame and ensures stability of flame.
On the other hand, combustion gas produced in a furnace circulates in the in-furnace area. The combustion gas in the furnace is exhausted therefrom as the combustion air and fuel enter into the furnace. The combustion gas still possesses a large amount of recoverable heat, and therefore, the combustion gas is exhausted to the ambient environment through a waste-heat recovery equipment, such as a heat exchanger, waste-heat boiler and the like. In general, such an equipment preheats combustion air or heats a fluid useful as a heat medium.
A part of in-furnace combustion gas forms an in-furnace re-circulation flow to be mixed with the combustion air and/or fuel injection flow, so that ignition of fuel is urged and a slow combustion reaction of a low oxygen density is promoted. Recently, mixing of combustion gas circulation flow with combustion air or fuel is considered to be important, since such mixing is effective to prevent a local heat of flame and restrict production of nitrogen oxide (NOx).
Mixing process and ratio of fuel injection flow, combustion air flow and combustion gas re-circulation flow are changed, depending on positions, structures and configurations of combustion air port and fuel injection port, and an arrangement and structure of combustion furnace, and so forth. Further, mixing control for various kinds of fluid in a furnace is closely associated with unexpected control parameters, such as change of furnace temperature, heat load, in-furnace circulation flow and so forth. Therefore, it is difficult to readily control the mixing process and ratio. Especially, as regards a combustion furnace which relatively often varies in heat load and furnace temperature in correspondence with its operating condition, mixing of combustion gas re-circulation flow with air and/or fuel might result in deterioration of combustion stability when the temperature of combustion air is lowered, and therefore, any countermeasure for overcoming this drawback is required. Thus, development of fuel feeding device is desired which enables optional and variable control of the mixing process and mixing ratio of fuel, combustion air and combustion gas, and which can normally optimize a combustion reaction in a combustion area.
Further, an extremely high-temperature air combustion method developed by the present applicant is known in the art, wherein combustion air is preheated up to a temperature equal to or higher than 800 degrees centigrade (deg. C.) and introduced into a mixing area or combustion area. A combustion mode of flame by the preheated air at a temperature equal to or higher than 800 deg. C. provides combustion stability in a combustion atmosphere with a wide range of air ratio, compared to a combustion mode of normal flame by air preheated to a temperature lower than 400 deg. C., or a combustion mode of transitional flame at a temperature of 400˜800 deg. C. The combustion stability in the extremely high-temperature air combustion method is considered to result from its combustion characteristics entirely different from the conventional method, owing to increase of reaction rate by a higher temperature of preheated air. Especially, when the combustion air or mixed gas for combustion is heated to a temperature higher than the self-ignition temperature of fuel, a combustion reaction without necessity of external ignition means can be realized in an ignition process. Further, flame failure can be prevented in spite of substantial increase of combustion air flow speed, so that the combustion air can be fed to a combustion area or mixing area as a high speed air flow. Furthermore, although increase of flame volume and decrease of flame brightness in accordance with the extremely high-temperature air combustion method are observed in the combustion area, phenomenon of local heat generation is restricted, so that a temperature field in the combustion area is rendered uniform.
Conventional research of radiation and convection heat transfer effects with respect to heating apparatus such as a tubular furnace is mainly directed to development of a combustion system which can generate a desirable temperature field in a combustion area while preventing a local overheat of a heated tube, or improvement of an arrangement and structure of the tubes and so forth. However, mixing of air and fuel generally tends to depend on control of temperature, flow rate, flow velocity, direction of air flow and the like, and therefore, characteristics of flame in a combustion area substantially rely on properties and fluid characteristics of air flow. For example, since fuel and air taking a combustion reaction in a mixing area almost entirely burn near a burner, a flame is merely formed near the burner, and therefore, it is difficult for the flame to reach a zone near a heated subject. On the other hand, if a feeding pressure of fuel is increased or a diameter of fuel nozzle is reduced for increasing a distance of travel of fuel fluid, a blowing speed of the fuel may be increased. However, the flow rate of fuel fluid is greatly smaller than that of air flow, and therefore, owing to a power of a large amount of air, the power of fuel fluid flow is de-energized to lose its power immediately after its injection. Thus, it is difficult to increase a distance of travel of the fuel fluid.
On the contrary, according to the extremely high-temperature air combustion method as set forth above, an air ratio and air-fuel ratio can be reduced and a flow rate of circulation flow of in-furnace combustion gas can be increased, whereby a slow combustion reaction can be maintained in a furnace and a temperature field in the furnace can be rendered in a uniform condition. However, in this kind of combustion method, a supply velocity of air flow tends to be set in a relatively high value. Therefore, the tendency that the control of mixing of fuel and air depends on control of air flow is more significantly revealed.
In addition, it has been found, in the extremely high-temperature air combustion method, that a mixing condition of a fuel injection flow, combustion air flow and in-furnace circulation flow is an important factor for controlling a combustion reaction, and therefore, it is necessary to focus on the mixing control of these three kinds of fluids upon adoption of an arrangement of apparatus. However, it is difficult in practice to surely control the mixing of these fluids in dependence on a conventional combustion skill in which an in-furnace circulation flow of combustion gas is mixed with a fuel or air flow within an in-furnace area of the furnace. Thus, development of a new combustion skill is desired, in which controllability of fuel flow itself delivered into a furnace is improved, and a position, diffusing manner and reach of flame can be controlled in dependence on control of the fuel flow, and further, controllability of mixing position and mixing ratio of fuel, combustion air and combustion gas can be improved.
It is therefore an object of the present invention to provide a fuel feeding apparatus and method which can improve controllability of mixing process and mixing ratio of fuel and combustion air.
Another object of the present invention is to provide a fuel feeding apparatus and method which allow a fuel and combustion gas to be optionally mixed, independently of control of in-furnace combustion gas re-circulation flow.
Still another object of the present invention is to provide a fuel feeding apparatus and method which can produce a fuel gas having new combustion characteristics.
Another object of the present invention is to provide a combustion system and method which can improve controllability of fuel flow entering a combustion area and which enable control of characteristics of flame by control of fuel flow, and further, a heating apparatus and method which can control properties of flame acting on heated subjects.